This is following the Mermaid storyline (What Storms May Come from June 2017). It is going to get really expensive for those who do not have a lot of coin makers.
If you look at the first post of that discussion thread SpiritWind had some good suggestions for gathering coins. Also, Shibuya has a nice list of the top ways to get coin in Information Nugget 12.
Mermaid thread:
https://forums.storm8.com/showthread...77#post1428377
Nugget 12 (coins):
https://forums.storm8.com/showthread...ugget-12-Coins
Good news for those who can not start now is that there is no limited time item. I have not seen a pop-up saying we would get anything for completing Part 1 in a specified time. We can take our time with completing this storyline.